The present invention relates to, among secondary batteries to be used as an operational power source for portable electronic equipment, or as a motor driving battery for an electric vehicle or a hybrid electric vehicle, etc., a lithium secondary battery which has small internal resistance and has good charge-discharge cycle characteristics, with a lithium transition metal compound being used as a positive active material.
In recent years, miniaturization to go with lighter weight is being investigated in an accelerated fashion with respect to electronic equipment such as a personal handy phone system, a video tape recorder, a notebook-sized personal computer, etc., and a secondary battery comprising a lithium transition metal compound as a positive active material, with a carbon material as a negative material, and an electrolyte obtained by dissolving a Li ion electrolyte in an organic solvent, has become common as the power source battery.
Such a battery is generally called a lithium secondary battery or a lithium ion battery, and since they are provided with larger energy density as well as with higher unit cell voltage of approximately 4V, attention is being paid to these batteries not only for the aforementioned electronic equipment but also as a motor driving power source for an electric vehicle or a hybrid electric vehicle which is under consideration for positive proliferation to the general public as a low pollution vehicle, in view of the recent environmental problems.
In such a lithium secondary battery, its battery capacity as well as its charge-discharge cycle characteristics (hereinafter called xe2x80x9ccycle characteristicsxe2x80x9d) heavily depends on the material characteristics of the positive active material to be used. The lithium transition metal compound to be used as the positive active material includes lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2), or lithium manganese oxide (LiMn2O4), etc. in particular.
Here, LiCoO2 as well as LiNiO2 comprises such features as a large Li capacity, a simple configuration, and excellent reversibility, and is provided with a two dimensionally layered configuration that is excellent in ion diffusion. On the other hand, however, as concerns LiCoO2, producing areas of Co are limited and it hardly is true that output quantity is abundant. Accordingly, these materials are expensive, and thus there is a cost issue and a problem that its output density is smaller compared with LiMn2O4. In addition, as concerns LiNiO2, synthesis of compounds of stoichiometric composition is difficult since the trivalent status of Ni is comparatively unstable, and in the case where detachment of Li becomes abundant, Ni will become subject to transition to bivalent status, emitting oxygen to constitute NiO, which creates problems such that the battery will stop functioning as a battery and there is a risk of battery explosion due to oxygen detachment.
On the contrary, LiMn2O4 has a feature that raw materials are inexpensive and larger output density as well as higher voltage is provided. However, in the case where LiMn2O4 has been used as a positive active material, there is a problem that repetition of charging-discharging cycle gradually decreases discharge capacity and good cycle characteristics will not become obtainable. It is deemed that the major cause of this is reduction of the positive capacity since crystal configuration changes irreversibly due to insertion and detachment of Li+.
Thus, a lithium transition metal compound such as LiCoO2, etc. respectively has both advantages and disadvantages together as a positive active material, and therefore, there are no rules which substances must be used, and it is deemed advisable that a positive active material which can show an appropriate feature for a particular purpose should be suitably selected for use.
Incidentally, regardless of the kind of positive active material, it is preferred in terms of characteristics of a battery that the internal resistance of the battery is small, and it is a common problem to all the positive active materials to be solved that resistance in a positive active material (namely electronic conduction resistance) should be reduced, or in other words, electronic conductivity should be improved for this reduction of the internal resistance. Particularly, in a lithium secondary battery of large capacity used as a motor driving battery for an electric vehicle, etc., it is very important to obtain large current output necessary for acceleration and gradeability, etc. to improve charging-discharging efficiency.
Under the circumstances, conventionally, trials to improve electronic conductivity by adding to a positive active material conductive fine grains such as acetylene black, etc. to reduce internal resistance of a battery have been conducted. This is becaused the above-described lithium transition metal compound is a mixed conducting body comprising both lithium ion conductivity and electronic conductivity together, but its electronic conductivity is not always strong.
However, there is a problem that addition of acetylene black causes reduction of filling quantity of a positive active material to reduce battery capacity. In addition, it is deemed that improvement of electronic conductivity is not unlimited since acetylene black is a kind of carbon and is a semiconductor. Moreover, acetylene black is voluminous and presents such a problem that it is difficult to handle when an electrode plate is to be produced. Accordingly, the volume of its addition is to be limited to an appropriate quantity, comparing and considering the advantageous effect of reduction of internal resistance, the disadvantageous effect of reduction of battery capacity, and the simplicity in production, etc.
Now, as described above, in the case where acetylene black has been added, acetylene black exists only on surfaces of particles of a positive active material, resulting in contributing to improvement of electronic conductivity among particles of positive active material, but not resulting in contributing to improvement of electronic conductivity inside a particle of a positive active material. Thus, conventionally, for improving electronic conductivity of a positive active material, attention was only paid to electronic conductivity among particles of a positive active material, but the relationship between diffusion of Li+ and electronic conductivity inside a particle of a positive active material at the time of battery reaction was not regarded as a problem.
In short, detachment of Li+ from a particle of a positive active material as well as insertion of Li+ to a particle of a positive active material is proceeded by diffusion of Li+ inside a particle of a positive active material, simultaneously accompanied by transfer of electrons taking place inside a particle of a positive active material, and at this time, if electronic conductivity inside a particle of a positive active material is low, diffusion of Li+ hardly is apt to take place and velocity of detachment and insertion of Li+, namely velocity of battery reaction, becomes slow, resulting in an increase in internal resistance, which was not taken into consideration at all.
The present inventors paid attention to this point, and considered in earnest to improve electronic conductivity of a positive active material itself so that diffusion of Li+ inside a positive active material may proceed well, thus reducing resistance of the positive active material itself, and at the same time, when a battery has been assembled without increasing volume of acetylene black to be added, internal resistance of that battery may be reduced, and as a result the present invention has been achieved.
According to an aspect of the present invention, there is provided a lithium secondary battery, comprising a lithium transition metal compound LiMeXOY, in which a portion of transition element Me is substituted by not less than two additional elements selected from the group consisting of Li, Fe, Mn, Ni, Mg, Zn, B, Al, Co, Cr, Si, Ti, Sn, P, V, Sb, Nb, Ta, Mo, and W to constitute LiMZMeXxe2x88x92ZOY (herein M represents substitution elements, and Mxe2x89xa0Me, and Z represents quantity of substitution) the LiMZMeXxe2x88x92ZOY being used as a positive active material.
In the present invention, not less than 2 kinds of elements are preferably selected as the substitution elements M among the above-described group of elements, particularly Li, Fe, Mn, Ni, Mg, Zn, Si, Ti, Sn, P, V, Sb, Nb, Ta, Mo, and W, and it is especially preferred that at least Ti is included. It is also preferred that a portion of the remaining transition elements Me in LiMZMeXxe2x88x92ZOY to include not less than two kinds of substitution elements M to be obtained this way is also preferably substituted further by at least one element selected among B, Al, Co, and Cr. Also it is preferred that in a lithium transition metal compound LiMZMeXxe2x88x92ZOY, Z/X, the ratio of the substitution quantity Z of substitution elements M and Me quantity X of the original transition elements, fulfills the condition of 0.005xe2x89xa6Z/Xxe2x89xa60.3.
Incidentally, as one of lithium transition metal compounds to be suitably used in the present invention, lithium manganese oxide, especially a lithium manganese oxide having a spinel configuration of cubic system, may be nominated. The average valence of substitution elements M to substitute a portion of manganese in such lithium manganese oxide is set at not less than 3 but not more than 4. Here, an average valence is an average value of ion valence of not less than two different substitution elements M in a positive active material. Here, in the case where lithium manganese oxide has been used, a substitution quantity Z preferably remains within a range of 0.01xe2x89xa6Zxe2x89xa60.5 and more preferably fulfills a condition of 0.1xe2x89xa6Zxe2x89xa60.3.
In addition, in the present invention, lithium cobalt oxide or lithium nickel oxide is suitably used as a lithium transition metal compound. In the case where such materials have been used, it is preferred that the average valence of substitution elements M to be substituted with a portion of cobalt or nickel in lithium cobalt oxide or lithium nickel oxide is 3. However, the case where all the substitution elements M have the ion valence of 3 is excluded. Here, the substitution quantity Z preferably remains within the range of 0.005xe2x89xa6Zxe2x89xa60.3, and further preferably fulfills the condition of 0.05xe2x89xa6Zxe2x89xa60.3.
LiMZMeXxe2x88x92ZOY to be used in the above-described lithium secondary battery of the present invention is composed by firing a mixed compound comprising salts and/or oxides having been prepared with a predetermined ratio in an oxidation atmosphere in a temperature range of 600xc2x0 C. to 1000xc2x0 C., for 5 hours to 50 hours. At this time, also suitably adopted is such a method that is conducted, dividing firing into not less than twice, with the firing temperature for the forthcoming step to be set higher than that for the previous step, and thus proceeding with the composition. Here, in the case where a plurality of firing steps is conducted, the final firing is to be conducted under a firing condition involving an oxidation atmosphere in a temperature range of 600xc2x0 C. to 1000xc2x0 C., for 5 hours to 50 hours.
In a lithium secondary battery of the present invention, a portion of transition element Me of a lithium transition metal compound LiMeXOY is substituted by not less than two elements to constitute LiMZMeXxe2x88x92ZOY, the LiMZMeXxe2x88x92ZOY being used as a positive active material. Here, M represents substitution elements, and substitution elements M are the one which are different from a transition element Me (Mxe2x89xa0Me), and Z represents quantity of substitution. Strictly, since not less than two kinds of substitution elements M are involved, the chemical formula of the positive active material is described as Li((M1)x1(M2)x2 . . . (Mn)xn)ZMeXxe2x88x92ZOY (herein, M1, M2, . . . , and Mn represent respectively different elements, and the total sum of x1 to xn is 1) for substitution by n-numbered kinds of elements. Incidentally, element substitution of the present invention involving such plural elements will be hereinafter called xe2x80x9ccomplex substitutionxe2x80x9d.
As substitution elements M, not less than two elements are selected from the group consisting of Li, Fe, Mn, Ni, Mg, Zn, B, Al, Co, Cr, Si, Ti, Sn, P, V, Sb, Nb, Ta, Mo, and W. These elements were determined by applying Hume-Rothery""s rule to an ionic radius introduced by SHANNON, et al which has been described in Acta Cryst. (1976). A32, 751, and for the ion radius of transition element Me to be substituted in a space group R(xe2x88x923)m (herein xe2x80x9cxe2x88x92xe2x80x9d represents rotation-inversion) or in Fd3m (a spinel configuration), a condition that the coordination number for oxygen is the same as that for the transition element Me and the average ionic radius of the substitution elements M remains within xc2x115% of the ionic radius of the transition element Me, and is not a radioactive element nor a gas, and not strongly toxic having been fulfilled so as to select a combination of elements. Here, as a transition element Me, Mn, Co, and Ni to be suitably used in the present invention are regarded as a standard.
An ionic radius of substitution elements M is referred to an average value of ionic radius of not less than 2 kinds of elements, and is determined in consideration of existence ratio of each element. In the present invention, it is preferable that all the ionic radii of the substitution elements M remains within xc2x115% of the ionic radius of the transition element Me, but in the case where such a condition may not be fulfilled, for example even in the case of the substitution element M1 numbered 1 with its ionic radius far larger than the range of xc2x115% of the ionic radius of the transition element Me, and the substitution element M2 numbered 2 with its ionic radius far smaller than the range of xe2x88x9215% of the ionic radius of the transition element Me, if an average ionic radius of the substitution elements M1 and M2 falls in the range of xc2x115% of the ionic radius of the transition element Me, complex substitution is feasible.
However, in the case where Li is used, Li can be used as a substitution element M, exceptionally, even when the above-described conditions on ionic radius are not fulfilled. The reasons of this are that other than the ionic radius of the above-described version of SHANNON, et al, there is also a version of Polling, et al, and there is a big difference in normal values for these versions, thus limiting consideration on only the ionic radius of Li is problematic in terms of character itself, and that Li is an original constitutional element and particularly in the LiMn2O4 system, Li is deemed to substitute the position of Mn, and further that it is experimentally possible to solid-solubilize Li.
Incidentally, as concerns substitution elements M, in theory, Li is to become a +1 valence ion, Fe, Mn, and Ni, Mg, and Zn are +2 valence ions, B, Al, Co, and Cr are +3 valence ions, Si, Ti, and Sn are +4 valence ions, P, V, Sb, Nb, and Ta are +5 valence ions, and Mo and W are +6 valence ions, and they all are elements to be solid-solubilized in LiMZMeXxe2x88x92ZOY. However, for Co and Sn, they can be +2 valence ions, and for Fe, Sb and Ti, they can be +3 valence ions and for Mn they can be +3 and +4 valence ions, and for Cr they can even be +4 and +6 valence ions.
Therefore, as seen in an actual positive active material, in the case where there exists a part of ionic valence subject to change in valence values due to various crystallographic deficiencies, in some cases there is a possibility that an average valence of substitution elements M might not coincide with theoretic valence value, e.g. 3.5 for lithium manganese oxide and 3 for lithium cobalt oxide as well as lithium nickel oxide, of a transition element Me prior to complex substitution.
For example, since Ti can exist comparatively stably under +3 valence condition in addition to +4 valence condition, in the case where Ti has been solid-solubilized in LiMZMeXxe2x88x92ZOY under the condition having such mixed atomic valence, the average valence of Ti falls in a range between +3 to +4. And as concerns Fe, since Fe remains equally stable under +2 and +3 valence condition and it is also known that the status of +4 valence exists stably in a certain chemical compound, the average valence of Fe in LiMZMeXxe2x88x92ZOY is to fall in a range between +2 to +4. In addition, similarly, also as concerns quantity of oxygen in LiMZMeXxe2x88x92ZOY, it may exist in deficit or in excess within a range to sustain a crystal configuration.
Incidentally, as a lithium transition metal compound to be used in the present invention, lithium manganese oxide, lithium cobalt oxide, and lithium nickel oxide may be nominated in particular. Here as concerns lithium manganese oxide, a lithium manganese oxide (LiMn2O4) having a spinel configuration of cubic system is suitably used. In LiMn2O4, one Mn in two units of Mn is in the state of +3 valence while the other Mn is in the state of +4 valence state. Accordingly, in complex substitution, two cases can be considered, namely a case where substitution elements M is used for substitution of Mn in this +3 valence state, and a case involving substitution of Mn in +4 valence state.
An average valence value of the substitution elements M is 3 in the case where complex substitution of +3 valence Mn takes place, but here at least elements to become ions with other than +3 valence is included in the substitution elements M. For example, such cases that two units of +3 valence Mn undergo complex substitution with one +2 valence Mg and +4 valence Ti, and two units of +3 valence Mn undergo substitution with one +1 valence Li and one +5 valence V can be nominated. And in the case where a +3 valence Mn undergoes complex substitution with such an element having other than +3 valence, it is permitted that the remaining +3 valence Mn is substituted with another +3 valence ion. Here, an average valence is referred to an average value of ion valence of not less than two different substitution elements M in a positive active material and is determined, putting their existence ratio under consideration.
Likewise, for the purpose that +4 valence Mn undergoes complex substitution, it is necessary that substitution has taken place with at least an element to provide a valence value other than +4 valence, and thereafter the remaining +4 valence Mn may be substituted with an element to provide the same +4 valence. In general, in complex substitution of LiMn2O4, at least it is necessary that the ionic valence of the substitution elements M numbered 1 is not more than 3 and the ionic valence of another substitution elements M is not less than 4, consequently resulting in the average valence of only substitution elements M to be ranged from not less than 3 to not more than 4, and the average valence value obtained from the substitution elements M after complex substitution inclusive of Mn being 3.5.
On the other hand, since the substitution elements M to make a portion of Co or Ni in lithium cobalt oxide (LiCoO2), and lithium nickel oxide (LiNiO2) undergo complex substitution is to provide an average valence value of 3, similarly in the above-described substitution of +3 valence Mn, the substitution elements M are to include elements to provide ions with at least other than +3 valence. Therefore, the case where all the substitution elements M have ionic valence value of 3 valence is excluded from complex substitution of the present invention.
In the case where a battery has been assembled using a positive active material which had undergone such complex substitution, there reveals an effect with remarkable reduction of internal resistance. This is deemed to be caused by that electronic conductivity is improved in the frame of lithium transition elemental composite compound (a portion exclusive of Li attributable to ionic conduction), and thus velocity of detachment and insertion of Li ions in battery reaction has become faster. And considering that the lattice constant gets small due to complex substitution, the improvement of electronic conductivity in this frame is presumed to heavily depend on that in the case where transition elements Me each other and/or substitution elements M are transition metal elements, the d orbital between substitution elements M and a transition element Me is apt to overlap, which makes it easier to smoothly proceed with the movement of electron by use of this d orbital.
In addition, repeating charge and discharge of a battery assembled by use of materials which have undergone complex substitution, no deterioration is observed, compared with the case involving use of materials which have not undergone complex substitution, and therefore, it is deemed that complex substitution does not negatively affect stability of the frame. Moreover, in LiMn2O4, as shown in the below-described embodiments, the cycle characteristics have been improved, thus it is deemed that complex substitution attributes to improvement of reversibility of crystal lattice associated with insertion and detachment of Li ions.
Incidentally, compared with the case where a portion of the transition element Me is substituted by another element (hereinafter, such substitution involving one element is referred to as xe2x80x9csingle element substitution according to complex substitution, such a problem that positive capacity might be reduced by larger volume of substitution in single element substitution can be avoided. Next, this example is explained by use of LiMn2O4, but it goes without saying that the explanation may be made to LiCoO2 and LiNiO2.
In the case where Mn3+ in LiMn2O4 has undergone single element substitution with an element having valence value of not more than two valence, e.g. one valence ion such as Li+, charge equivalent to +2 valence value, being a difference of charge with Mn3+, will be in short, thus for the purpose of maintaining electrical neutrality of materials, two units of Mn3+ will be changed to Mn4+. Thus, consequently, one Li+ will be substituted with Mn3+ and solid-solubilized, resulting in reduction of approximately three units of Mn3+.
Here, in LiMn2O4, it is deemed that, at the time of charging, electrical neutrality of materials is maintained by compensating shortage of charge due to detachment of Li+ with Mn3+ being changed to Mn4+, and at the time of discharging reverse reaction takes place. In short, the quantity of Mn3+ in LiMn2O4 determines the positive capacity, and a quantity of Li+ corresponding to Mn3+ attributes charging and discharging reaction. Therefore, for the purpose that Li+ is detached from a crystal lattice or inserted into a crystal lattice, it will become necessary that a change in valence value takes place in cations other than Li+, namely substitution elements M and/or transition element Me.
However, in the previous embodiment, Li+ which was substituted with Mn3+ has not undergone change in valence value, consequently Mn3+ remains in short by three units. Therefore, 3 units of Li+ will not attribute to charging and discharging reaction. In short, consequently there arises a problem that the positive capacity is reduced in excess of quantity of substitution. Such a problem similarly takes place in single element substitution involving +2 valence ions.
On the other hand, in complex substitution of the present invention, substitution elements M are to be narrowed to Li, Fe, Mn, Ni, Mg, Zn, Si, Ti, Sn, P, V, Sb, Nb, Ta, Mo, and W (hereinafter these substitution elements M are referred to as xe2x80x9csubstitution elements group within a reduced rangexe2x80x9d), and not less than two elements are arranged to be selected, then in addition to an effect that improves electronic conductivity, the above-described problem that the positive capacity is reduced in excess of quantity of element substitution is avoided.
In short, when ions with +1 valence or +2 valence and ions with +4 to +6 valence are combined, as concerns shortage of positive charge caused by solid-solubilizing ions with +1 valence or +2 valence, the charge is not compensated by change of Mn3+ to Mn4+, but ions with +4 to +6 valence are solid-solubilized and compensated, thus without reducing the positive capacity as a result of reducing the number of Mn3+ in excess of substitution quantity, Mn can undergo substitution.
For example, in the case where two units of Mn3+ are substituted by one Li+ and one V5+, reduction of positive capacity is limited to a reduced volume of two units of Mn3+, and it will become possible to make quantity of reduction of Mn3+ lesser than reduction by three units of Mn3+ in the case where one Mn3+ has undergone single element substitution with one Li+. In addition, in the case where two units of Mn3+ have been substituted with one Mg2+ and one Ti4+, reduction of positive capacity is limited to reduction covering two units of Mn3+, and is less than reduction of four units of Mn3+ in the case where two units of Mn3+ have been substituted with two units of Mg2+. Thus, reduction quantity of Mn5+ is equivalent to substitution quantity of elements, and accordingly such event that reduction in positive capacity exceeds substitution quantity is to be avoided.
Here, in complex substitution, when at least Ti is arranged to be included as substitution elements M, a remarkable effect of improvement on electronic conductivity is obtainable and preferable. In addition, Ti can be effectively used to prevent a drop in positive capacity, which is preferable.
In LiMZMeXxe2x88x92ZOY including not less than two kinds of substitution elements M obtainable when complex substitution using elements among the above-described substitution elements group within a reduced range, a portion of remaining transition elements Me may further be substituted with at least not less than one element selected from B, Al, Co, and Cr. In this case, complex substitution involving at least three kinds of element is to take place.
These elements such as B and Al, etc. exist in LiMZMeXxe2x88x92ZOY as ions with +3 valence in theory. But, as described above, in actual positive active materials, the ion valence value does not always have to correspond with the theoretic valence values. Ions with +3 valence are substituted with Mn3+ one on one, therefore, decrease in positive capacity is the same as the quantity of substitution, and decrease in positive capacity not less than the quantity of substitution does not take place, and on the other hand, the said ion attributes to improvement of electron conductivity of a positive active material itself. Incidentally, in the case where LiMn2O4 is used, an effect that its crystal configuration is made reversible toward insertion and detachment of Li+ is provided.
Next, substitution quantity Z in complex substitution is explained. In the present invention, it is preferred that Z/X, the ratio of the quantity Z to be substituted by substitution elements M to the quantity X of the original transition element Me fulfills the condition of 0.005xe2x89xa6Z/Xxe2x89xa60.3. When Z/X is less than 0.005, resistance of a positive active material does not drop, and improvement in cycle characteristics rarely appears. In short, no effects of complex substitution appear. On the other hand, when Z/X is more than 0.3, in synthesis of a positive active material, production of a different phase is admitted through powder X-ray diffraction method (XRD), and a single phase material was not obtainable. In a battery, such a different phase only increases the weight of a positive active material and does not attribute to battery reaction, thus it goes without saying that production of a different phase at the time of synthesis together with entry to the battery should be avoided.
Positive-active-material-wise, in particular, when LiMn2O4 has been used, the substitution quantity Z preferably fall within a range of 0.01xe2x89xa6Zxe2x89xa60.5, and further preferably to falls in a range of 0.1xe2x89xa6Zxe2x89xa60.3, and when LiCoO2 as well as LiNiO2 is used, the substitution quantity Z preferably falls within a range of 0.005xe2x89xa6Zxe2x89xa60.3, and further preferably to falls in a range of 0.05xe2x89xa6Zxe2x89xa60.3, and within the respective preferable ranges of the substitution quantity Z, there remarkably appears an effect of improvement of electronic conductivity of a positive active material, which is preferable.
Incidentally, when elemental substitution by not less than one element selected from B, Al, Co, and Cr further took place as well after complex substitution, the total substitution quantity (Z+W) of substitution quantity Z of substitution elements M selected from a group of substitution elements within a reduced range, and the substitution quantity (to be referred to as xe2x80x9cwxe2x80x9d) of B and Al, etc. is required to fulfill a relationship of 0.01xe2x89xa6Z+wxe2x88x920.5.
Incidentally, LiMZMeXxe2x88x92ZOY to be used in a lithium secondary battery of the present invention, is composed by firing a mixed compound comprising salts and/or oxides of each element (substitution elements M as well as Li and transition element Me) having been prepared with a predetermined ratio in oxidation atmosphere at a temperature range of 600xc2x0 C. to 1000xc2x0 C., for 5 hours to 50 hours, and thus a single phase product can be obtained. Here, an oxidation atmosphere is referred to as an atmosphere having partial pressure of oxygen with which generally a sample inside a furnace is brought into oxidation reaction. In synthesis of LiCoO2 as well as LiNiO2, it is preferable that the partial pressure of oxygen is set at not less than 10%, and, in particular, air atmosphere and oxygen atmosphere, etc. are applicable.
Incidentally, when the firing temperature is as low as less than 600xc2x0 C., a peak showing residue of raw material, e.g. peak of lithium carbonate (Li2CO3) in the case where Li2CO3 is used as a lithium source, is to be observed in XRD chart of fired product, and no single phase products can be obtained. On the other hand, when the firing temperature is as high as more than 1000xc2x0 C., high temperature phase is produced in other than a compound of the targeted crystal system, and a single phase product will become no longer obtainable.
In addition, firing may be conducted, being divided into not less than twice. In that case, it is preferable that the firing is proceeded with the firing temperature for the forthcoming step to be set higher than that for the previous step, and the final firing is to be conducted under a firing condition involving an oxidation atmosphere at a temperature range of 600xc2x0 C. to 1000xc2x0 C., for 5 hours to 50 hours. Thus, in the case of firing taking place twice, for example, applying this condition of second firing temperature as well as firing period, the product obtainable when synthesis has been conducted with the temperature for the second firing to be set at not less than the temperature for the first firing features steeper projection in the peak shape in the XRD chart than with the product obtainable when a single firing yields, and as a result improvement of crystallinity can be planned.
A salt for each element will not be limited in particular, but it goes without saying that those having intensive purity and further being inexpensive as raw materials are preferably to be used. Accordingly, such carbonate, hydroxide, and organic acid/salt that do not produce harmful decomposition gas at the times of elevation of temperature or filing are preferably used. However, nitrate, hydrochloride, and sulfate, etc. are not always unusable. Generally, in synthesis of LiCoO2 and LiNiO2, it is known that synthesis temperature goes down with usage of salts instead of oxides as raw materials. Here, as concerns raw materials on Li, usually an oxide Li2O is chemically unstable, and thus it is rarely used.
As the foregoing, implementation of complex substitution of the present invention will make improvement in electronic conductivity of a positive active material easier to plan, providing preferable electric characteristics, and resulting in decrease in internal resistance of a battery. In addition, the problem that positive capacity might be reduced by larger volume of element substitution in single element substitution which conventionally used to be problematic in single element substitution, is to be solved, and reduction of positive capacity is to be suppressed to the extent equivalent to quantity of element substitution. At the same time, as concerns LiMn2O4, reversibility of crystal configuration for insertion and detachment of Li+ is improved, thus cycle characteristics as a battery are improved. Accordingly, decrease with the passage of time in battery capacity due to repetition of charging and discharging is controlled.
Reduction of internal resistance and reservation of positive capacity, and increase in cycle characteristics are planned in such a battery, which is used as a motor driving power source for an EV or an HEV in particular, consequently providing with an excellent effect that predetermined running performance such as acceleration performance as well as slope-climbing performance, etc. is maintained, and continuous running distance per for charging is kept for long.
Incidentally, other materials to be used for production of a battery are not limited whatsoever, and conventionally known various materials can be used. For example, as a negative active material, an amorphous carbon material such as soft carbon or hard carbon, or carbon material such as artificial graphite such as high graphitized carbon material, etc. and natural graphite, etc. are used.
And as an organic electrolyte a carbonic acid ester family such as ethylene carbonate (EC), diethyle carbonate (DEC), and dimethyle carbonate (DMC), and the one in which one or more kinds of lithium fluoride complex compound such as LiPF6, and LIBF4, etc. or lithium halide such as LiClO4 as an electrolyte are dissolved in a single solvent or mixed solvent of organic solvents such as propylene carbonate (PC), xcex3-butyrolactone, tetrahydrofuran, and acetonitrile, etc., can be used.